The present invention relates to isocyanate compositions comprising a) at least one isocyanate and b) sulfur. The present invention also relates to a process of dissolving sulfur in an isocyanate, reacting the isocyanate with suitable co-reactants to form polyurethanes and/or polyureas and polyurethanes and/or polyureas containing finely divided sulfur particles embedded in the polymer matrix. This invention also relates to a process for producing encapsulated fertilizers comprising applying A) at least one coat of an isocyanate-reactive component to fertilizer particles, and applying at least one coat of the present isocyanate compositions to fertilizer particles, thus forming sulfur containing polyurethane encapsulated fertilizer particles; B) premixing sulfur containing isocyanate and an isocyanate-reactive compound prior to applying it; C) applying first the sulfur containing isocyanate followed by applying the isocyanate-reactive compound; and D) applying the sulfur containing isocyanate compound and using humidity or water to form a sulfur containing organic polyurea or polyurethane/polyurea. The present invention also relates to the encapsulated fertilizers produced by these processes.
Elementary sulfur is used in many applications, i.e., as a vulcanizing agent in the rubber industry, as a component in the preparation of thiokols (organic polysulfides used, i.e., as sealants, adhesives, impregnation agents and insulation materials), as part of match making compositions, as a raw material for black powder production (mixture of sulfur, charcoal and potassium nitrate). Sulfur has also been used to encapsulate fertilizers to control the release of plant nutrients. In this case, the sulfur is applied to the fertilizer particles in either a molten form or dispersed in an organic liquid.
In many of these applications, it would be desirable to use sulfur in a diluted form either as a solution in a liquid organic compound or in form of very finely divided, homogeneously dispersed particles.
Elementary sulfur is insoluble in water and almost all liquid organic compounds such as, for example, alcohols, esters, ethers, ketones, etc, except for some organic compounds that have sulfur already chemically bonded to the organic molecules, i.e., CS.sub.2 (carbon disulfide) and certain thiols, alkyl sulfides, sulfoxides, sulfones and sulfonic acids. (See Basic Principles of Organic Chemistry, John D. Roberts and Majorie C. Caserio, Published by W. A. Benjamin, Inc., 1965, New York, Amsterdam; Sulfur Solubility in Pure and Mixed Organic Solvents, Ind. Eng. Chem. Res., 1988, 27, pp. 485-491; and "Solubility of Elemental Sulfur in a Number of Organic Solvents" by G. S. Frolov, V. I. Lazarev, I. S. Antsypovich and T. V. Epishkina translated from Zhurnal Prikladnoi Khimii, Vol. 48, No. 8, pp.1853-1855, August, 1975, Original Article submitted Oct. 14, 1974.
Commercial particulate fertilizers are produced and marketed in several different particle types, i.e., granular, pelletized, dusts, pilled, and prilled fertilizers. Also, they may be formed of inorganic substances, organic substances, or combinations thereof. The isocyanate compositions of the present invention can be applied to any of these types of particulate fertilizers.
In order to be effective in promoting plant growth, fertilizers must contain some amount of water soluble plant nutrients. These are typically in the form of water soluble compounds of nitrogen, phosphorus and potassium, alone or in combination, and often in conjunction with other elements such as, for example, calcium, boron, magnesium, zinc, chlorine, etc. Such particular fertilizers can be made of a single component, e.g., urea, ammonium nitrate, potassium chloride, etc., or of multiple components often mixed with inert water soluble or water insoluble materials as in common fertilizers designated as 6-6-6, 4-6-4, 10-10-10, 20-20-5, 14-16-0, 5-20-20, and the like. In addition, specialized fertilizers may contain optional additives such as herbicides, insecticides, trace elements, iron salts, sulfur, etc. The isocyanate compositions of the present invention can be applied to any of these fertilizers.
Historically, particulate fertilizers possessed a number of known defects, the most notable being the too rapid release of soluble plant food, causing phytotoxicity and the rapid depletion of the plant nutrients by leaching. Other problems included tendencies to cake and form dust. These problems are well documented in previous patents that professed solutions to one or more of the known defects, including U.S. Pat. Nos. 3,475,154, 3,259,482, 3,264,088, 3,264,089, 4,711,659, 4,772,490 and Japanese Patent 52-38361.
A number of slow release coatings for particulate fertilizers have previously been proposed. One of the methods to achieve slower release of the fertilizer has been the application of liquid sulfur which is usually done in a rotating horizontal drum. The fertilizer is usually preheated to the granules. However, when the sulfur comes in contact with the fertilizer granules, it solidifies rapidly and thus, is deposited in the form of chunks and flakes on the fertilizer surface. Although the release of nutrients is slowed down by using this treatment, the fertilizer is only partially encapsulated and the sulfur is inherently brittle and does not provide the necessary abrasion resistance and protection when these sulfur coated fertilizers are packaged, transported and, finally, applied by a spreader. Thus, a lot of the sulfur may have already delaminated from the fertilizer particle and the slow release properties are further diminished. The process of sulfur coating urea particles is described in U.S. Pat. No. 3,342,577 and was developed in the late 1960's by the Tennessee Valley Authority (TVA) as an economical system for reducing the rate of dissolution when urea particles are applied to the soil as fertilizer. This process requires high levels of sulfur, which reduces the nutrient analysis of the fertilizer particles, and even then, imperfections remain in the coating, making it necessary to apply a sealant coating, which, for example, can be composed of a mixture of 30% polyethylene resin in a 70% bright stock mineral oil.
Coatings in which preformed polymer resins are applied from solvents have been described in, for example, U.S. Pat. Nos. 3,475,154 and 3,264,089. The use of solvents creates a vapor hazard and environmental problem as the products are dried and the solvent evaporation step can result in pinhole imperfections in the coatings when applied.
U.S. Pat. No. 4,369,055 tried to facilitate degradation of coating materials while maintaining the function for controlling the dissolving-out rate by dispersing inorganic powders such as sulfur and talc in a low molecular weight olefin polymer. However, the disclosed coating materials do not provide good attrition resistance and are not completely degraded and thus remain intact in the soil.
Polyurethane coatings as disclosed in U.S. Pat. Nos. 4,711,659 and 4,969,947 require that the substrate contains a minimum quantity of reactive --NH.sub.2 groups. Thus, these are not applicable to all fertilizer compositions for which slow release properties may be desirable.
U.S. Pat. No. 5,538,531 also describes controlled release fertilizers and a method for their production. These controlled release fertilizers have a central mass of particulate fertilizer which contains at least one water soluble plant nutrient surrounded by a plurality of coatings. The inner coating comprises the reaction product of (A) an aromatic polyisocyanate or derivatives thereof which contain about 1.5 to 3 NCO groups per molecule and an NCO group content of 10 to 50% by weight, and (B) a polyol having from 2 to 6 hydroxyl moieties and at least one alkyl moiety containing from about 10 to 22 carbon atoms. An outer coating is also necessary. The outer coating consists essentially of an organic wax having a drop melting point of between 50 and 120.degree. C. The encapsulated fertilizers of U.S. Pat. No. 5,538,531 do not, however, contain sulfur in any form.
U.S. Pat. No. 5,645,624 describes an encapsulated fertilizer using first a layer of rapidly biodegradable aliphatic polyester and polyurethane compound followed by a slowly biodegradable layer based on cellulose derivatives and hydrocarbon compound to control the release rate of the fertilizer.
Advantages of the present invention include the fact that the isocyanate compositions of the present invention provide an improvement in the production of particulate fertilizers by allowing the sulfur to be distributed homogeneously onto the surface of the fertilizer particles, instead of being deposited as large, irregular chunks on the surface of the fertilizer particles. Upon reaction of the sulfur containing liquid polyisocyanate with a suitable Zerewitinoff active hydrogen containing compound (organic compounds containing --OH, --NH, --NH.sub.2 or --SH groups) solid polyurethanes and/or polyureas are formed that contain sulfur in an evenly distributed, finely divided form throughout the polymer matrix.